A dead man presents a puzzle for Rügen forensic scientist Leona Pirell: The man died from cyanide poisoning. The poison was found in a hollowed-out tooth, which initially puts the victim's dentist in the crosshairs of investigating detective Peer Boström. However, Leona does not believe in his guilt and sets out to uncover the truth. In the process, she stumbles upon a closely guarded secret involving the settlement of an old debt.